As oil and gas are extracted from producing wells, sand and heavy oils that have flowed through the perforations accumulate. These are too heavy to flow to the surface along with the usual fluids produced by the well at normal production rates, and tend to accumulate in low-lying areas as shown in FIGS. 1 and 2. Additionally, drilling muds used during the drilling process are generally heavier than the reservoir fluid, and tend to also segregate to low-lying areas of the well. Finally, proppant used during reservoir fracturing operations is not always completely removed. The accumulation problem in particularly severe where the trajectory of the well 10 is at or close to horizontal in the producing reservoir 12 and sumps 14 are present. Deposits 16 in these regions reduce the effective cross-section of the well 10 with a corresponding decrease in flow area 18 and therefore increase the pressure drop of the production fluids. In order to maintain or increase the production of such a well, it is necessary to remove this fill. Conventional methods of fill removal (or cleanout) involve high-pressure jetting through coiled tubing (CT) to mobilize the fill around the cleaning tool and sweep it to the surface by slowly pulling the CT up, the flow of jetting fluid and production fluid carrying the loosened fill to the surface. This high pressure mobilization jetting does increase the Bottom Hole Pressure (BHP) of the reservoir though, so it is only applicable to wells in formations that can sustain a full hydrostatic column (or foam column, in the case of foam clean-outs) and the increased pressure due to jetting. An example of one such technique is the PowerCLEAN service offered by Schlumberger.
In most well cleanout applications, the reservoir pressure is high enough, and the rock permeability low enough, to allow increase of pressure in the well while performing cleanout operations. In others, foam can be used to sweep the fill up. However there are still many wells that either cannot hold a foam column, or where foam use is restricted due to logistics reasons (e.g. procurement and disposal of N2 foam). In these situations, the only existing cleanout solution is a concentric coiled tubing (CCT) service to power a downhole jet pump. Using CCT implies a high use of power liquid to move the fill out and is limited in length by the weight of the coil-in-coil assembly.
Examples of CCT techniques can be found in U.S. Pat. No. 2,548,616, U.S. Pat. No. 5,033,545, U.S. Pat. No. 5,269,384, U.S. Pat. No. 5,375,669, U.S. Pat. No. 6,263,984, U.S. Pat. No. 6,015,015, U.S. Pat. No. 6,497,290, U.S. Pat. No. 6,640,897, U.S. Pat. No. 6,712,150, U.S. Pat. No. 5,503,014, and WO 2005085580 A.
Embodiments of the invention aim to provide an alternative to CCT techniques while also extending the depths at which clean-out operations can be performed. An embodiment of the invention is based on the use of a downhole pump that is powered by a cable running inside the tubing conveyance.